1. Field of the Invention
The present invention generally relates to ultrasonic bonding and more particularly to a support structure used for securing a semiconductor device during the bonding.
2. Related Art
As global markets demand smaller power semiconductor devices for mobile application at lower costs, manufacturers seek to improve the process and make it as efficient as possible. These cost reduction strategies involve using less raw copper material. This forces the creation of highly populated lead-frames used to transport the semiconductor packages through the front-end manufacturing process. Highly populated lead-frames contain many rows and columns of semiconductor devices connected to a matrix by small and thin tie bars. Because of the density of the devices on the lead-frame and their inherently small sizes, properly constraining the packages during the large ultrasonic wire or ribbon bonding process becomes very difficult. Poor clamping leads to an unreliable process and poor quality parts.
FIG. 1 shows one type of conventional clamping that uses an array of thin flexible clamp fingers 102 (only one shown) to constrain specific points on a lead-frame 104 during ultrasonic bonding. Lead-frame 104 is constrained by the spring action of clamp fingers 102. Clamp fingers 102 are designed to flex a specific amount during the clamping operation to ensure even clamping force along lead-frame 104 in the presence of tolerance stack up between components. Due to the flexibility of the clamp fingers, a large amount of tip deflection occurs over the surface of lead-frame 104 (shown by the portion between the two arrows). This movement combined with the high force applied to the clamps can damage the lead-frames by leaving visible marks on the frames. As a result, the placement of the clamps are constrained to areas encapsulated in the final package or areas that are cut away and not included in the final package (for cosmetic reasons). Clamp placement may also be constrained to places on the lead-frame large enough to handle the deflection of the clamp fingers. Furthermore, the use of small and light clamp fingers to secure specific points on the lead-frame can cause unwanted resonance problems during bonding due to the many modes of the clamp fingers in the frequency spectrum close to the bonding frequency. This can cause instability and inconsistency in the bonding process as well as rendering an application impossible to bond with a given set of tooling.
Thus, there is a need for a way to bond lead-frames without the disadvantages of conventional clamping discussed above.